1. Field of the Invention
The present invention relates to a projector having a device for measuring an angle of inclination between the projection optical axis of a projection device and a projection surface, and a method for correcting a trapezoidal distortion.
2. Description of the Related Art
As projectors have become smaller in size and higher in performance due to rapid advances in solid-state pixel display device technology, projectors for projecting images are being widely used and have become more attractive for use as large-size display apparatuses to replace home display-type TV sets.
However, since a projector, unlike a display-type TV set, projects an image onto a projection surface such as a screen or a wall, the displayed image tends to be distorted, depending on the relative relationship between the projection optical axis of the projector and the projection surface. The specification etc. of Japanese Patent Laid-open Publication No. 281597/97 discloses a process of adjusting the angle of a liquid crystal display unit. The process has the step of detecting the installation angle of a liquid crystal projector and the step of detecting the distance between the liquid crystal projector and the projection surface onto which an image is projected. According to the disclosed processes, the angle of the liquid crystal display unit needs to be mechanically adjusted.
There has also been disclosed a projector apparatus that detects the distance from the projection lens to a screen, calculates the angle of inclination, then automatically corrects the distortion based on the detected distance. See the specification etc. of Japanese Patent Laid-open Publication No. 355740/92, No. 81593/2000, and No. 122617/2000 for details.
In these prior art systems or processes in which the distance to the screen is detected, however, the distance measuring device is provided independently of the projector. The projected images are not utilized. Further, the measured distance and the actual distance to the screen do not always coincide, if the screen is surrounded by a frame projecting forwardly toward the projector, or the screen is set back away from the projector.
The specification etc. of Japanese Patent Laid-open Publication No. 169211/2001 discloses a process of correcting a distortion by projecting a beam spot onto a curved screen from an angle-controllable laser pointer. According to the disclosed process, a spot image is generated and projected onto a screen by the projector, and the beam spot and the projected spot image are captured by a camera for measuring their positions. The spot image is moved until it overlaps with the beam spot, then the coordinate of the pixel of the spot image in the frame memory is replaced with the coordinate of the beam spot and stored in a coordinate transform parameter memory. Although this process is excellent for acquiring an angle accurately, it needs a means to control the different angles (positions) of the laser pointer, and also requires a complicated system configuration, since it is necessary to employ a laser pointer and a digital camera having a two-dimensional array of imaging elements.
According to another disclosed projector that sells in the market, the inclination of the projector in the vertical direction, which easily causes a distortion, is detected by a gravity sensor, when a screen is set vertically. The distortion is corrected depending on the detected inclination. See the specification etc. of Japanese Patent Laid-open Publication No. 5278/2003 for details. However, the projector cannot correct the distortion correctly, if the screen is not set vertically or the screen is inclined in the horizontal plane with respect to the projection optical axis of the projector.
Once the angles of inclination of optical axis of the projector apparatus are detected with regards to the screen in the vertical and horizontal directions, an image can be projected without distortion from the projector onto the screen by means of conventional techniques such as the conversion of the coordinates of a frame memory of the projector, etc.